U.S. patent number 4,525,027 [Application Number 06/648,334] was granted by the patent office on 1985-06-25 for single mode optical fibers.
This patent grant is currently assigned to Nippon Telegraph & Telephone Public Corp.. Invention is credited to Takao Edahiro, Akio Kawana, Tetsuo Miya, Katsunari Okamoto.
United States Patent |
4,525,027 |
Okamoto , et al. |
June 25, 1985 |
Single mode optical fibers
Abstract
In a single mode optical fiber comprising a core made of glass
and a cladding surrounding the core and made of glass, when a
difference .DELTA. between refractive indices n.sub.1 and n.sub.2
of the core and cladding is expressed by (n.sub.1
-n.sub.2)/(n.sub.1).times.100%, the refractive indices n.sub.1 and
n.sub.2 are determined to satisfy a relation 1.0<.DELTA.<3.6
and a diameter of the core is determined according to this value of
.DELTA..
Inventors: |
Okamoto; Katsunari (Mito,
JP), Edahiro; Takao (Mito, JP), Kawana;
Akio (Komae, JP), Miya; Tetsuo (Mito,
JP) |
Assignee: |
Nippon Telegraph & Telephone
Public Corp. (JP)
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Family
ID: |
15034496 |
Appl.
No.: |
06/648,334 |
Filed: |
September 5, 1984 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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518772 |
Aug 2, 1983 |
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192259 |
Sep 30, 1980 |
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Foreign Application Priority Data
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Oct 8, 1979 [JP] |
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54-130449 |
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Current U.S.
Class: |
385/123;
385/142 |
Current CPC
Class: |
G02B
6/02238 (20130101) |
Current International
Class: |
G02B
6/10 (20060101); G02B 005/172 () |
Field of
Search: |
;350/96.29,96.30,96.34 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Niizeki, "Single Mode Fiber at Zero-Dispersion Wavelength", Proc.
of Topical Mtg. on Int. & Guided Wave Optics (Salt Lake City),
Jan. 1978, pp. MB1-1 to MB1-4. .
Miya et al., "Ultimate Low-Loss Single-Mode Fibre at 1.55 .mu.m",
Electron. Lett., vol. 15, No. 4, Feb. 1979, pp. 106-108. .
White et al., "Zero Total Dispersion in Step-Index Monomode Fibres
at 1.30 and 1.55 .mu.m", Electron. Lett., vol. 15, No. 13, Jun.
1979, pp. 396-397. .
Okoshi et al., "Single-Polarisation Single-Mode Optical Fibre With
. . . ", Electron. Lett., vol 16, No. 18, Aug. 1980, pp.
712-713..
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Primary Examiner: Lee; John
Attorney, Agent or Firm: Helzer; Charles W.
Parent Case Text
This application is a Continuation-In-Part application of U.S.
patent application Ser. No. 518,772 filed Aug. 2, 1983 which in
turn was a Continuation application of parent application Ser. No.
192,259 filed Sept. 30, 1980 (both now abandoned).
Claims
What is claimed is:
1. A single mode optical fiber comprising a core having a diameter
of 2a and made of glass having a refractive index of n.sub.1, and a
cladding surrounding the core and made of glass having a refractive
index of n.sub.2 wherein when a refractive index difference .DELTA.
is expressed by n.sub.1 -n.sub.2 /n.sub.1 .times.100, said
refractive indices n.sub.1 and n.sub.2 are determined to satisfy a
relation 1.0<.DELTA.<3.6 and the diameter 2a of said core is
selected to have a value in a range of 4.8 to 3.0 microns as
determined by the value of .DELTA..
2. The single mode optical fiber according to claim 1 wherein said
core and said cladding are essentially made of quartz glass.
3. The single mode optical fiber according to claim 2 wherein said
core is made of quartz glass incorporated with a dopant which makes
larger the refractive index of said core than that of said
cladding.
4. The single mode optical fiber according to claim 3 wherein said
dopant is a member selected from a group consisting of germanium,
phosphor, titanium, aluminum, tantalum, zirconium, lead, barium,
and gallium.
Description
BACKGROUND OF THE INVENTION
This invention relates to a single mode optical fiber, and more
particularly a single mode optical fiber in which a core is
surrounded by a cladding.
Various efforts have been made to improve the single mode optical
fiber to meet requirements raised by the development of optical
communication and in recent years improved single mode optical
fibers have been developed which in addition to ordinary
characteristics of light weight, noninductive and no cross-talking
have a lower loss than a transmission line utilizing metal
conductors such as a millimeter wave guide and can transmit data
over a wider frequency band. A long distance ultra wide frequency
band optical communication system operating at a frequency of 1 GHz
and without using any repeating station, is now being planned by
using optical fibers.
It has been theoretically determined that it is necessary to reduce
the total dispersion to be less than .+-.1 ps/km/nm and to reduce
the transmission loss to be less than 0.5 dB/km in order to realize
such long distance ultra wide band transmission. Considering the
transmission characteristics of present day optical fibers from
this standpoint of view, the ultra wide band long distance
transmission involves certain problems for the following
reason.
In an optical fiber, light entering into the core transmits
therethrough by repeatedly being reflected at the interface between
the core and the cladding, and deterioration of the transmission
characteristics caused by the dispersion of light can be analyzed
as follows: (1) multimode dispersion.
This is caused by the fact that the propagation constant
representing the propagation state of the light varies nonlinearly
with respect to the angular frequency of the light and that as the
angular frequency increases, higher order modes appear in the light
propagating mode so that group velocities vary for different modes.
(2) material dispersion (.delta..sub.M).
This is caused by the fact that the refractive index of glass
constituting the optical fiber varies nonlinearly and that the
material dispersion .delta..sub.M of a single mode optical fiber is
shown by the following equation ##EQU1## where n.sub.1 represents
the refractive index of the core, .lambda. the wavelength of light
and C the light velocity in vacuum. (3) waveguide dispersion
.delta..sub.W.
The waveguide dispersion is determined by the relationship between
the propagation constant .beta. and the angular frequency of light
.omega. and is expressed by the following equation in the case of
the single mode optical fiber ##EQU2##
In the case of a multimode optical fiber it is necessary to take
into consideration the three dispersions described above, but the
invention is directed to a single mode optical fiber for the
purpose of decreasing signal distortion by minimizing as far as
possible the dispersions. Hence, it is not necessary to consider
the multimode dispersion but only the material dispersion and the
waveguide dispersion must be considered. Accordingly, in the case
of the single mode optical fiber a sum (.delta..sub.M
+.delta..sub.W) of the material dispersion .delta..sub.M and the
waveguide dispersion .delta..sub.W constitutes the total dispersion
.delta..sub.T and the frequency bandwidth f in which signals can be
transmitted without distortion is expressed by the following
equation.
In a typical prior art optical fiber the diameter of the core
2a=9.0 microns, the refractive index of the core n.sub.1 =1.46319,
and the difference in the refractive indices of the core and
cladding ##EQU3## Examples of such single mode fiber are described
in a K. Daikoku et al. paper entitled "Direct Measurement of
Wavelength Dispersion in Optical Fibres--Difference Method",
Electronics Letters, 1978, Vol. 14, No. 5, pages 149-151.
With such prior art optical fibers, when the wavelength of light
varies in a range of 0.9-1.6 microns, for example, the material
dispersion varies in a range of from +66 ps/km/nm to -25 ps/km/nm
and the difference in the refractive index and the core diameter
has values described in the preceeding paragraph, the waveguide
dispersion .delta..sub.W varies in a range of from 4 ps/km/nm to 13
ps/km/nm. For this reason, the total dispersion .delta..sub.T
varies in the positive and negative directions about .lambda.=1.43
microns. Accordingly, when the optical fiber is operated at a
wavelength .lambda.=1.43 microns at which the total dispersion
.delta..sub.T becomes zero the distortion of the signal becomes
minimum so that transmission over a long distance at a wide
bandwidth becomes possible. However, if the wavelength differs from
1.43 microns even slightly, the total dispersion .delta..sub.T
would increase greatly with the result that the frequency bandwidth
f shown in equation (3) rapidly decreases.
Accordingly, with the prior art optical fiber, even when the total
dispersion .delta..sub.T is made to be equal to .+-.1 ps/km/nm
suitable for long distance transmission, the usable wavelength
width is at most from 1.41-1.43 microns. For this reason, for long
distance wide bandwidth transmission, even when the wavelength is
divided or multiplexed for the purpose of improving the
transmission efficiency, it has been impossible to operate the
optical fiber in the wavelength range described above.
SUMMARY OF THE INVENTION
Accordingly, it is a principle object of this invention to provide
an improved single mode optical fiber having wider band
transmission characteristics than the prior art optical fibers thus
enabling wavelength division multiplex transmission.
According to this invention, there is provided a single mode
optical fiber of the type comprising a core having a diameter of 2a
and made of glass having a refractive index of n.sub.1, and a
cladding surrounding the core and made of the glass having a
refractive index of n.sub.2, characterized in that, when a
refractive index difference .DELTA. is expressed by n.sub.1
-n.sub.2 /n.sub.1 =100, the refractive indices n.sub.1 and n.sub.2
are determined to satisfy a relation 1.0<.DELTA.<3.6 and that
the diameter 2a of the core is determined according to this value
of .DELTA..
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings:
FIG. 1 is a perspective view showing the basic construction of a
single mode optical fiber embodying the invention;
FIG. 2 is a graph showing the refractive index distribution in the
radial direction of the optical fiber shown in FIG. 1;
FIG. 3 is a graph showing the relationship between the wavelength
and dispersion characteristics .delta..sub.W, .delta..sub.M and
.delta..sub.T of the optical fiber shown in FIG. 1; and
FIG. 4 is a graph showing the relation between the refractive index
difference and the wavelength of the light, the core diameter, the
total dispersion, and the transmission loss characteristics
obtained by simulation.
DESCRIPTION OF THE PREFERRED EMBODIMENT
A single mode optical fiber 10 shown in FIG. 1 comprises a core 11
made of quartz glass or the like, and a cladding 12 made of similar
material and surrounding the core. Denoting the refractive index of
the core 11 by n.sub.1, that of the cladding by n.sub.2 and the
radius of the core by a, the refractive index distribution of the
core as viewed in the radial direction of the optical fiber 10 is
shown in FIG. 2. As can be noted from this distribution, the
optical fiber of this type has a so-called step type refractive
index distribution. The refractive index difference between the
core 11 and the cladding 12 is expressed by the following equation.
##EQU4##
According to this invention, this refractive index difference
.DELTA. is selected to lie in a range of from 1% to 3.6% and the
diameter 2a of the core 11 is selected to have a suitable value in
a range of 4.8 to 3.0 microns as the .DELTA. varies (i.e., the
diameter 2a would decrease as .DELTA. increases). By using such
values of the refractive index difference .DELTA. and the core
diameter 2a, it is possible to obtain a novel optical fiber having
a total dispersion .delta..sub.T of less than .+-.1 ps/km/nm and a
theoretical value L of the transmission loss closely related to the
refractive index difference .DELTA. of less than 0.5 dB/km. The
optical fiber embodying the invention can use a wavelength width of
0.05 to 0.23 microns depending upon the values of the refractive
index difference .DELTA. and the core diameter 2a. As a
consequence, with a wavelength width of 0.05 micron, a two
wavelength multiplexing becomes possible assuming a spacing of the
wavelength multiplexing of 230 .ANG. where the wavelength shift of
a semiconductor laser caused by temperature variation is 200 .ANG.
and the spectral width of a light source is from 20 to 30
.ANG..
The lower limit of the refractive index difference .DELTA. is
determined by the fact that the bandwidth of the wavelength is 0.05
micron so that as .DELTA. decreases the wavelength width gradually
decreases thus making it difficult to accomplish wavelength
multiplexing. The upper limit of the refractive index difference
.DELTA. is determined by the fact that as the core diameter becomes
smaller than 3 microns, the coupling between adjacent cores as well
as the coupling between the core and the light source become
imperfect so that losses at these portions increase. With the
present day coupling construction, when the core diameter is 3
microns, it is possible to limit the coupling losses to be less
than 10 dB. Moreover, with this construction, it is possible to
improve by 3 to 4 times the photo-coupling efficiency with the
light source since the permissible light receiving angle increases
due to the increase in the refractive index difference between the
core and the cladding.
Furthermore, when the optical fiber is used in a submarine cable
the light is perfectly confined within the core due to a high
refractive index difference, thus decreasing the increase in the
transmission loss caused by hydrostatic pressure in contrast to the
prior art optical fibers.
FIG. 3 is a graph showing the relationship between the wavelength
of the light to be transmitted through the single mode optical
fiber of this invention and the material dispersion .delta..sub.M,
the waveguide dispersion .delta..sub.W and the total dispersion,
i.e., the sum of .delta..sub.M and .delta..sub.W. In this case, the
core 11 and the cladding 12 were essentially made of quartz galass.
In making the core a germanium dopant was used for making the
refractive index thereof larger than that of the cladding. Thus,
the refractive index difference was .DELTA.=2.4%, and the diameter
of the core 11 was 2a=3.46 microns. As can be noted from the
characteristics shown in FIG. 3, the total dispersion .delta..sub.T
was made to be less than .+-.1 ps/km/nm for a wavelength range of
.lambda.=1.48-2.0 microns.
By using an electronic computer, we have made a simulation
calculation for the purpose of confirming the operation and the
advantageous effect of the single mode optical fiber according to
this invention. The following table shows the values of the
diameter 2a of the core 11 and a wavelength range .lambda..sub.b in
which the total dispersion .delta..sub.T lies in a range of .+-.1
ps/km/nm when the refractive index difference .DELTA. varies
between 0.8 and 3.6%.
TABLE ______________________________________ .DELTA. (%) 2a (.mu.m)
.lambda..sub.b (.mu.m) ______________________________________ 3.8
2.94 1.63 -1.65 3.6 3.00 1.55-1.61 2.4 3.46 1.40-2.00 2.0 3.68
1.46-1.62 1.5 4.12 1.42-1.50 1.0 4.78 1.40-1.44 0.8 5.19 1.37-1.42
0.6 5.62 1.38-1.40 ______________________________________
FIG. 4 is a graph showing the low dispersion region (a region
bounded by .delta..sub.Tu and .delta..sub.Td curves) in which
.delta..sub.T is less than .+-.1 ps/km/nm for the refractive index
difference .DELTA., the diameter 2a of the core 11 and a low loss
region of less than 0.5 dB/km (a region bounded by curves L.sub.u
and L.sub.d).
These characteristic curves show that the value of 2a should be
determined in a hatched area shown in FIG. 4 in order to obtain a
single mode optical fiber having characteristics presently desired,
i.e., a total dispersion of less than .+-.1 ps/km/nm, and a
transmission loss of less than 0.5 dB/km. As above described, the
values of .DELTA. and 2a that determine this area are
.DELTA.=1-3.6% and 2a=4.78-3.00 microns. For example, to mainly
transmit light essentially having a wavelength of about
.lambda.=1.5 microns, the refractive index difference
.DELTA.=2.4-1.5% and the diameter of the core 11 2a=3.46-4.12
microns are adequate. In a quartz glass optical fiber, it is known
that the transmission loss is minimum near a wavelength of about
1.5 microns, the advantageous effect of this invention can be
efficiently manifested near this wavelength. The fact that the
transmission loss becomes minimum near the wavelength of 1.5
microns is described in Miya's paper entitled "Ultimate Low Loss
Single Mode Fiber at 1.55 .mu.m", Electronics Letters, Vol. 15, No.
4, pages 106-108, Feb. 15, 1979.
Furthermore, as shown in FIG. 4, where .DELTA.=1.1% and 2a=4.62
microns, the usable wavelength bandwidth is 1.40-1.45 microns,
meaning possibility of two wavelength multiplexing. Especially,
where .DELTA.=2.05% and 2a=3.67 microns, the usable wavelength
bandwidth is 1.46-1.69 microns meaning a possibility of 10
wavelength multiplexing.
Although in the foregoing embodiment the core and cladding were
made of quartz glass having a predetermined refractive index,
incorporation of germanium acting as a dopant into the quartz glass
increases the refractive index n.sub.1. Thus, from equation (4) it
can be noted that it is possible to increase the refractive index
difference .DELTA.. In addition to germanium, the dopant or
additive may be selected from the group consisting of phosphor,
titanium, aluminum, tantalum, zirconium, lead, barium, and
gallium.
A specific way of determining semidiameter a of the core is as
follows:
To determine the total dispersion .delta..sub.T, the following
formula has been found applicable. ##EQU5##
In equation (5) the first term represents the material dispersion
(.delta..sub.M) and the second term represents waveguide dispersion
(.delta..sub.W). As can be determined from equation (7), the
diameter a of the core is included in v as an implicit function.
Thus, the relationship between the core diameter a and the
refractive index difference .DELTA. for the purpose of the present
invention of making the total dispersion nil (i.e. .delta..sub.T
=0) over a wide wavelength region can be obtained through use of a
computer, but it is not possible to explicitly indicate the
solution.
The relationship between b and v can be found from the solution of
the following equation ##EQU6## as indicated by the Chart 2 below:
##STR1##
With respect to the waveguide dispersion .delta..sub.W indicated as
##EQU7## in formula (5) above, it is reached from the formula (2)
of this specification ##EQU8## through the following analytical
steps.
Since w=kc in which k is the wavenumber in vacuum
(k=2-.pi./.lambda.) and c is the light velocity, .delta..sub.w can
be expressed as: ##EQU9## An application of .beta.=kn.sub.1
.sqroot.1-2.DELTA.(1-b) to formula (11) above results: ##EQU10##
Where k.sup.2 n.sub.1.sup.2 /.beta..perspectiveto.kn.sub.1
##EQU11## When formula (13) is differentiated, ##EQU12## From
formulas (11) and (14) ##EQU13## Since v=kn.sub.1 a
.sqroot.2.DELTA., formula (15) can be expressed ##EQU14##
With respect to the material dispersion of a fiber made of quartz,
it is widely known that the wavelength dependency can be obtained
by utilizing Sellmeier's equation. Thus, the total dispersion
.delta..sub.T can be calculated based on equation (5) by applying
the .DELTA., .lambda.c and a of a single mode optical fiber whose
initial diameter a is selected employing FIG. 4. Specifically, the
wavelength dependency of .delta..sub.T is calculated to check
whether or not .vertline..delta..sub.T .vertline..ltoreq.1 ps/km/nm
stands in the low loss wavelength region of .lambda.=1.4 to 1.7
.mu.m, and in the case the condition is not satisfied, .lambda.c is
varied to seek the c and b which satisfy the condition with the
given .DELTA..
* * * * *